1. Field of the Invention
The present invention relates to an engine and gear drive combination.
2. Discussion of the Related Art
In internal combustion engines a variety of components including auxiliaries are driven from the crankshaft, the drive nowadays usually being via a belt or chain (both referred to as a belt for convenience) from a crankshaft sprocket or drive wheel. Some of these components, such as the camshafts and fuel injection pumps (for a diesel or fuel injection engine) are termed low-speed components as they are driven at less than crankshaft speed, specifically half-speed for a 4-stroke engine, while others, for example the water pump and alternator are high-speed components being driven above crankshaft speed. These different speed requirements have necessitated the provision of two separate drive belt systems, one for the low-speed components and the other for the high-speed components, as the crankshaft sprocket or wheel for the low-speed system, having to be relatively small, would not be suitable for the high speed system.
The use of two timing systems causes an increase in the length of the engine making it less compact, and additionally there are increased maintenance requirements as the tension in each belt system will have to be adjusted individually and eventual replacement of both belts may be needed. The slow speed belt can have a tensioner device while the alternator of the high speed belt can be movable for belt tensioning.
Further, the speed reduction at the engine camshaft has meant that relatively large camshaft drive wheels (or sprockets) are needed, leading to an increased height of the engine (for an overhead camshaft design) thereby causing possible problems in installing the engine below an engine head or bonnet. The problem may be mitigated by altering the valve angles but this may adversely affect engine performance.
Additionally, in engine designs up until now, it has been customary to operate with set (fixed) valve timing and fuel injection timing relative to a crankshaft datum. This, however, is not conducive to optimum engine performance for running over a range of different operating and load conditions on the engine. There have been proposals previously for phase adjustment in these timings during engine running but these have proved over complex and/or ineffective.